1. Field of the Invention
The present invention relates to a lens moving speed control device for focusing in a zoom lens for use with an image pickup device such as a video camera.
2. Related Background Art
Conventionally, a variety of lens types have been provided for a zoom lens for use with a video camera. FIG. 1 shows the configuration of a conventionally typical zoom lens. 1 is a front lens group for making the focus adjustment by moving in an optical axis direction, 2 is a variator lens group for the magnification changing, 3 is a compensator lens group for retaining the imaging plane fixed in the magnification changing, in which the variator lens group 2 and the compensator lens group 3 are moved in the magnification changing while retaining a predetermined relation.
4 is a lens group for the image formation, and 5 is an imaging plane on which an image pickup element such as a CCD is arranged. It is the common practice to have a fixed interlock relation between the variator lens and the compensator lens, a so-called front focus lens which makes the focusing with the front lens group as shown in FIG. 1 is mechanically interlocked therewith using a mechanical cam ring.
On the other hand, there is known a lens of the type generally referred to as an inner focus or rear focus which makes the focusing with a lens group behind the variator lens, unlike the front focus lens.
FIG. 2 shows an example of rear focus zoom lens of this type, in which the front lens group is fixed therein, and the variator lens group 2 and the compensator lens group 3 are interlocked using a mechanical cam ring in a certain relation, like the front focus lens group as shown in FIG. 1. 4 (or a part of 4) is a lens group for the focusing. When the focus lens group is disposed behind a magnification changing lens in this manner, it is necessary to move the lens group 4 during the zooming even if the object distance is constant, and the position occupied by the lens group 4 in accordance with the focal length will change depending on the object distance.
FIG. 3 shows the focusing positions of the lens group 4 of FIG. 2 dependent upon the focal length as represented for several object distances. In the same figure, the abscissas axis indicates the focal length, in which "W" is a wide edge of the zoom lens, and "T" is a tele edge thereof. Note that the zoom lens as denoted hereinafter indicates a variator lens. The ordinates axis indicates the position of the lens group 4, in which each curve (or straight line) as indicated by 6 to 10 represents the focusing position depending on the focal length for a certain fixed object distance. For example, 6, 7, 8, 9 and 10 show focusing loci with object distances of 50 cm, 1 m, 2 m, 10 m and .infin., respectively.
When the lens group 4 is greatly fed out (or moved to the object side) in wide as a position indicated by 11, the focusing is possible at a distance immediately before the lens.
Another inner focus lens configuration example is shown in the figure. In this case, a lens group 1, a variator lens 2, and an iris 12 are moved in a direction of the arrow in changing the magnification from wide to tele, in which 4A is a fixed lens group and 4B is a focus lens group.
In this case, since there is no compensation lens group (corresponding to the lens group 3 in FIG. 2) interlocked in a predetermined relation behind the variator lens 2, the graph as shown in FIG. 3 is represented as in FIG. 5. For example, 13, 14, 15, 16 and 17 show the focusing loci at the object distances of 50 cm, 1 m, 2 m, 10 m and .infin., respectively.
The lens system having the characteristics as shown in FIG. 5 may be, in addition to an optical system of FIG. 4, such that the lens group 1 and the iris 12 are fixed and the lens group 2 only is moved in zooming in the same figure.
In this way, the lens referred to as an inner focus or rear focus has the advantage that it allows for the focusing in a shorter range than with the front focus lens, and contributes to the realization of a smaller lens depending on the lens type, but conversely, the position of focal plane may shift even with no change of the object distance in zooming, whereby it is necessary to correctly retain the relation as shown in FIGS. 3 and 5 in accordance with the distance not to cause out of focus during the zoom operation.
The method of preventing such out of focus from occurring with the zoom operation will be discussed in the following.
First, a combination with an automatic focus adjusting device of the TTL method will be described. For example, in an automatic focus adjusting device for a video camera, a method is generally known in which the peak of high frequency component of an image pickup signal from CCD or the like is used as a focusing position.
FIG. 6 shows the principle thereof. Taking the position of lens group for the focus adjustment in the abscissas axis and the high frequency component (focus voltage) of image pickup signal in the ordinates axis, a peak is reached at a position as indicated by the arrow in the figure, this position A being a focusing lens position.
FIGS. 7A and 7B show an example of how to obtain the focus voltage F as shown in FIG. 6.
FIG. 7A shows an actual field of view for the image pickup, in which 20 is an angle of view, 18 is a range (distance measuring frame) for picking up a video signal for the automatic focus adjustment, and 19 is an object.
In FIG. 7B, (a) shows an object within the distance measuring frame. (b) is a video signal (Y signal) of the object as shown in (a). Differentiating this signal, it has a wave form as shown in (c), and further taking the absolute value thereof, it has a wave form as shown in (d). A signal (e) is obtained through the sample-and-hold of the signal (d), and the level F thereof is a focus voltage.
FIG. 8 shows a block configuration diagram of a camera having a combination of such an automatic focus adjusting device and an inner focus lens. An image pickup element such as a CCD is disposed at an imaging position 5. Then a luminance signal Y is produced by a camera circuit not shown, and the information within the distance measuring frame is input to a focus detection (AF) circuit 21. In the AF circuit, the focus voltage is obtained with the aforementioned method, whereby based on this focus voltage and the direction of driving the focusing lens 4B or the sign of change in the focus voltage value caused by the driving, the focusing is judged, and if unfocused, the kind of out of focus is determined to be front focus or rear focus, and based on this determination result, a focus lens driving motor 22 is driven in a predetermined direction.
However, it is quite difficult to make an accurate positional control of the lens group 4B during the zooming and remove the out of focus, based only on such a distance measurement result of the automatic focus adjusting device using an image pickup signal.
That is, in the near-tele, far distance as shown in FIG. 5, there is a great movement of the focus lens group (i.e., upward inclination of the curve) for minute variation in the abscissas axis (minute positional variation of the variator). This shows that the out of focus state will easily result unless the distance measuring operation of the automatic focus adjusting device is performed rapidly and correctly.
Generally in the automatic focus adjusting device using an image pickup signal, the interval at which the distance measuring result is obtained is about 1/60 second at minimum as it is restricted by the field period.
Accordingly, there is a limit of the "rapidness", and all the distance measuring results are not obtained correctly for every 1/60 second in practice, and further, it is difficult to optimize the selectable moving speed of the focus lens at a time.
From the above considerations, it will be found that with a configuration as shown in the block diagram of FIG. 8, it is impossible to remove the out of focus in zooming unless there is provided means for reducing the zoom speed to an extremely low value particularly at the tele end.
Thus, a second means for removing the out of focus in zooming is contemplated in which the data of focusing locus as shown in FIGS. 3 and 5 is stored within a CPU in the form of expression, positional data or focus lens moving speed relative to the zooming speed, and the driving contents of both actuators for a zoom motor for the driving of variator lens and a focus motor for the driving of focus lens are determined from the memory contents as well as the information as to the focal length in zooming and the position of focus lens.
For example, when a map indicated with the focal length in the abscissas axis and the focus lens position in the ordinates axis as shown in FIGS. 3 and 5 is subdivided into small regions I, II, III, IV, . . . . . as shown in FIG. 9, and the moving speed of the variator lens in the abscissas axis is assumed to be V.sub.V, a method for storing the moving speed V.sub.F of the focus lens for each region has been disclosed in Japanese Laid-Open Patent Application No. 1-280709. In this case, the speed V.sub.F is obtained by differentiating the focusing locus passing through a central point of a small region. When the zooming is performed using V.sub.V and V.sub.F, the focusing can be maintained under the condition where the object distance is not changed during the zooming in a tele to wide direction, while the focusing is difficult to maintain totally in zooming in a wide to tele direction.
This is because the interval between each curve corresponding to a respective object distance which is dispersed over a wide position range at the tele end, e.g., each locus representing the focusing point from .infin. to 1 m, converges into a narrow range on the wide side owing to the relation of depth of field (i.e., the positional difference of focusing point decreases), as can be clearly seen from FIGS. 3 and 5. On the other hand, if the width of depth is obtained on such a map, based on a permissible circle of confusion with a certain iris value, it is in most cases substantially constant irrespective of the focal length in the inner focus lens. Accordingly, in the focusing state on the wide side, the focusing point (which lies within a depth but is unknown precisely) is not necessarily located on a true focusing locus, whereby even if the focus lens position at that time is known, it does not necessarily trace a correct curve due to the divergence of the curve on the tele side, so that the focusing point on the tele side can not be determined accurately.
As above described, it is difficult to prevent out of focus from occurring during the zooming even if the object distance is not changed with the inner focus lens, whereby in general, the out of focus is removed with a zooming algorithm consisting of the aforementioned two methods, i.e., the automatic focus adjusting device of the TTL type using the image pickup signal and the memorized focusing locus of the map.
For example, Japanese Laid-Open Patent Application No. 3-41878 discloses a method in which the direction of out of focus is determined to be front focus or rear focus, based on a phase of the variation of focus voltage when the CCD is displaced minutely in an optical axis direction by a piezoactuator, and based on this determination result, the speed content for correcting for the front focus or the rear focus as well as the aforementioned speed for each small region are memorized to thereby select an optimal speed among them.
In this case, it is possible to remove entirely the out of focus in zooming in a wide to tele direction, but there are some problems with the cost, the size and the consumption power due to the use of a piezoactuator.
In order to detect the out of focus direction when unfocused, in addition to a method for detecting the front focus or rear focus with the so-called modulation method in which an optical member is displaced minutely in an optical axis direction by a piezoactuator, there is known a so-called trial method in which the focus lens is moved temporarily in either one direction, and further moved depending on whether the change of focus voltage is increasing or decreasing at that time.
Generally, the trial method for the detection of front focus or rear focus will take a longer time for the determination, and is more difficult to make a rapid determination, than the modulation method.
As will be clearly seen from the above discussion, in order to remove the out of focus arising in a lens of the inner focus type by using the trial method for determining the direction of front focus or rear focus, it is requisite to resolve the following problems:
1 As the focusing lens needs to change continuously the position in accordance with the focal length and the object distance, it is necessary to make a considerably minute setting or switching of the focus motor speed to perform the operation of "moving the focus lens to the front focus or rear focus side temporarily".
2 In order to obtain adequately the variation of focus voltage, the setting of the focus motor speed in consideration of the change in the depth of focus in F number is necessary. Therefore, the method of remembering the service speed for each small region as previously described was insufficient.